1. Field of the Invention
The present invention pertains, in general, to a method for removing sludge accumulated in a crude oil tank on storage of crude oil in an oil refinery, and recovering oil therefrom. More specifically, the present invention pertains to a method for removing sludge in a crude oil storage tank by use of thermal oil, such as atmospheric residue, vacuum residue and vacuum gas oil yielded from an oil refinery, and recovering oil having various applications by separating inorganics from the removed sludge.
2. Description of the Prior Art
Generally, the term xe2x80x9ccrude oilxe2x80x9d refers to oil recovered from below the earth""s surface which remains untreated or unrefined. The problem begins when contaminants settle down on the bottom of oil storage tanks. Contaminants come from various sources and some of the contaminants are indigenous to the crude oil itself. Sludge is formed when naturally occurring solids as well as rust from piping and tank walls, and higher molecular weight hydrocarbons are separated from lighter hydrocarbons, and sink to storage tank bottoms. Such sludge is present in the form of stable emulsions consisting of long-chain paraffin, asphaltenes, inorganics and water.
In many countries, a crude oil storage tank of an oil refinery must be regularly examined every 8-10 years because of danger of oil spills by leaks of the tank. As such, the sludge accumulated in the crude oil storage tank is cleaned by several methods. However, when oil spill accidents occur despite such prior examination, the sludge should be cleaned prior to repairing the crude oil storage tank. Also, the accumulated sludge reduces the storage capacity. In addition, when the sludge is not removed for long periods of time and accumulated to the height of the crude oil-outlet, it overflows the tank and thus flows into the next process, thereby negatively affecting the total processes. In particular, if the level of the sludge is higher than that of the crude oil outlet, large quantities of the sludge flow into the process, thus charge pump trip of the crude oil occurring. Inflow of the sludge results in shortening cleaning cycle period of a heat exchanger and deactivation of a catalyst, and reducing of throughput attributed to frequent filtering operation in catalyst process. Therefore, every oil refinery invests much time and money for treatment of the sludge, but there is no other method except that the sludge-accumulated tank is opened, and then the sludge is removed. Hence, tank desludging processes (TDSP) have been under vigorous study.
Said sludge treatment techniques are classified into removal and discharge of the sludge from the tank; and separation and recovery of oil from discharged sludge. The discharge technique is exemplified by manners of introducing humans or bulldozer into the tank (SuperMacs), melting the sludge by installation of a heating tube or use of steam, and milling and discharging the sludge by ejecting the stream of crude oil under high pressure using a nozzle mounted on the tank.
SuperMacs technique mainly used in Korea, as stated above, discharges the sludge by ventilating harmful hydrocarbon gas filled in the tank after the tank is opened, and operating a vacuum pump-mounted bulldozer in the tank by an operator. The discharged sludge is melted by the steam, and then centrifuged to remove rust dregs and mud, thus recovered oil being back introduced into another tank. However, at that time, vicious cycles, in which the recovered oil becomes solidified over again and thus sludge is formed, are repeated. Furthermore, since said procedure is carried out in an opened system, environmental problems such as air pollution occur, and cleaning time (80-100 tons of sludge/day) is so long that it is undesirable in the applicatory aspect.
In the case of Statia Terminals (Oil and Gas Journal, Feb. 20, 1995) in Canada, the sludge is melted using a gas heater by installing a heating tube to the tank, but installation of the gas heater in the crude oil-storage area is complicated and dangerous, so the gas heater is difficult to apply to an oil refinery.
A method for crushing and discharging the sludge through ejection of a crude oil stream under high pressure using a nozzle has been used in Japan and Western countries. In Japan, a manner for crushing the sludge by ejecting the oil under high pressure through tens of ejection nozzles mounted on the tank roof, so called crude oil washing, has been employed. In Western countries, a manner for crushing the sludge by mounting a high pressure pump of large positive displacement and a nozzle to the manhole of the tank has been widely used.
In this regard, U.S. Pat. No. 5,078,799 discloses that crude oil is taken from an upward region of the tank, pumped upwardly to a suction pump-pressure pump unit located, for example, on top of a floating roof of the tank, the pressurized crude oil taken from the top portion of the tank being reintroduced through a preferably rotating liquefaction lance to a region in the vicinity of the sedimented sludge. In the above process, the pressure is converted into hydrodynamic energy. Preferably, a plurality of such lances are used, arranged with respect to each other to form eddies or vorteces, to establish flow patterns within the tank, thereby liquefying the sludge and rendering it into pumpable fluid condition for removal from the tank. Further, U.S. Pat. Nos. 4,945,933, 5,019,016 and 5,460,331 disclose an apparatus for dispersing sludge contained in a crude oil storage tank by use of a crude oil circulator, capable of forming jet flow by a plural number of rotatable nozzles positioned within the tank. However, said patents have the disadvantages that introduction of the sludge-dispersed crude oil causes fouling during distillation process and negatively affects other processes, and long periods of time are required to install the nozzle and the pump, and also the installation thereof is complicated.
When the discharged sludge is wasted, it is classified as malignant waste so that treatment cost becomes high, thus the sludge should be preferentially treated. Hence, a method for recovering the oil from the sludge is very important. Techniques for separating and recovering oil from the discharged sludge include a mechanical separation method using a centrifuge as disclosed in U.S. Pat. No. 4,990,237 and a thermal separation method as well as a solvent extraction or a sonic separation method.
Further, U.S. Pat. No. 5,085,710 refers to a process for separating and removing the hydrocarbon, water and solid components of sludge deposited in an oil storage tank which comprises: introducing a sufficient amount of a nonionic surfactant in an aqueous solution to form a layer of the solution above the sludge layer, said nonionic surfactant comprising C8-12 alkylphenol-ethylene oxide adduct, said nonionic surfactant being present in a quantity sufficient to separate hydrocarbon components from the sludge without forming an emulsion; adding a diluent, immiscible with the aqueous layer, for extracting the hydrocarbons; and separately draining the diluent layer and aqueous layer from the tank. But, said patent suffers from the disadvantage of using unfavorable chemicals in terms of the protection of the environment.
Also, U.S. Pat. No. 6,069,002 discloses a method for using a biological system to recover a hydrocarbon volume and minimize waste from a sludge oil in an oil storage tank, which includes the steps of: introducing a treatment solution comprising a volume of water, nitrogen source, phosphates, and microorganisms into the storage tank to break down the sludge oil into, in part, the hydrocarbon volume; heating the treatment solution; vigorously mixing and agitating the contents of the storage tank including the sludge; breaking down the sludge with the microorganisms; and removing the hydrocarbon volume from the storage tank as a useable and processable hydrocarbon fluid.
In addition to said techniques, various methods for the sludge removal and its treatment have been presented but are disadvantageous in terms of removal efficiency, washing time and economic efficiency. Accordingly, studies on related techniques have been carried out.
Leading to the present invention, the intensive and thorough research for using atmospheric residue, vacuum residue and vacuum gas oil, yielded from a distillation tower of an oil refinery, as thermal oil, and utilizing the heat contained in said oils, carried out by the present inventors aiming to avoid the problems encountered in the prior arts, resulted in the finding that sludge in a crude oil storage tank is melted and then discharged within short periods of time using said thermal oil, which is then left to stand in said crude oil storage tank or an additional separation tank, inorganics settled from said sludge being removed to a level of tens of ppm, therefore oil recovery being 95% or higher, thereby using said recovered oil as marine fuel or a feed for up-grading processes in an oil refinery.
Therefore, it is an object of the present invention to provide a method for removing sludge and recovering oil therefrom, capable of minimizing air pollution by removing the sludge from the crude oil tank under a closed system.
It is another object of the present invention to provide a method for removing sludge and recovering oil therefrom, this operation capable of being easily performed with minimal initial investment cost, drastically reducing washing time of a crude oil storage tank and continuously removing the sludge.
It is a further object of the present invention to provide a method for removing sludge and recovering oil therefrom, in which thermal oil of high temperature yielded from a crude oil-distillation process is used to remove the sludge, thus increasing economic efficiency of the process.
It is still another object of the present invention to provide a method for providing recovered oil as marine oil or feed for secondary treatment processes such as up-grading processes of heavy oil.
In accordance with an embodiment of the present invention, there is provided a method for removing sludge and recovering oil therefrom, comprising (a) introducing thermal oil yielded from an oil refining process, to sludge accumulated in a crude oil storage tank, at a volume ratio of 3:1 to 20:1, said thermal oil being selected from the group consisting of atmospheric residue, vacuum residue and vacuum gas oil; (b) melting the sludge until a mixture of said thermal oil and the sludge has a homogeneous temperature distribution with or without stirring said introduced thermal oil and sludge; (c) settling inorganics by allowing the mixture of said thermal oil and sludge to stand in said storage tank or an additional separation tank connected to said storage tank; and (d) recovering oil from the mixture of said thermal oil and sludge.